In industrial fluid bed cracking of hydrocarbon feedstocks, it is the practice, because of the rapid loss in catalyst activity and selectivity, to continuously add fresh catalyst regularly, usually daily, to an equilibrium mixture of catalyst particles. If metals, such as nickel and vanadium, are present in the feedstock, they accumulate almost completely on the catalyst, thus drastically reducing activity, increasing coke and hydrogen production, and reducing selective conversion to gasoline. In such cases of high metal content, catalyst replacement additions may have to rise significantly.
Fluid cracking catalysts consist of small microspherical particles varying in size from 10 to 150 microns, with a majority in the 40-105 micron range, and represent a highly dispersed mixture of catalyst particles that have been present in the unit for as little as one day, while others have been there for as long as 60-90 days or more. Because these particles are so small, no process has been available to remove old catalysts from new. Therefore, it usually is customary to withdraw 1 to 10% or more of equilibrium catalyst containing all of these variously aged particles just prior to addition of fresh catalyst particles, thus providing room for the incoming fresh material. Unfortunately, the 1 to 10% of equilibrium catalyst withdrawn contains, among other things, a likewise 1-10% of the very expensive catalyst added the day before, 1-10% of the catalyst added 2 days ago, 1-10% of the catalyst remaining of the catalyst added 3 days ago, and so forth. Therefore, when removing equilibrium catalyst, it is unfortunate that a very large proportion of withdrawn catalyst still represents very expensive and still very active catalyst.
Catalyst consumption can be very high. The cost associated therewith, especially when high nickel and vanadium are present in any significant amount greater than, for example, 0.1 ppm in the feedstock can, therefore, be very great. Depending on the level of metal content in feed and the desired operating catalyst activity and metal level desired in circulating equilibrium catalyst, tons of catalyst must be added daily. For example, the cost of a catalyst at the point of introduction to the unit can rise as high as $2,000/ton or greater. As a result, a unit consuming 20 tons/day of catalyst would require expenditures each day of at least $40,000. For a unit processing 40,000 B/D this would represent a processing cost of $1/B or 2.5 cents/gallon, for catalyst use alone. The above cost is more or less typical for a residual processing operation.
In addition to catalyst costs, an aged high nickel and vanadium ladened catalyst can also bring about a reduction in yield of valuable and preferred liquid fueld products, such as gasoline and diesel fuel, and instead, produce more undesirable, less valuable products, such as dry gas and coke. As if these two losses are not enough, a high level of nickel and vanadium on catalyst can, in addition, also act to accelerate catalyst deactivation, thus reducing operating profits even more.
Because of this required daily addition of fluid cracking catalyst, there results immediate and complete mixing of these microspherical particulates both fresh in performance and low in contaminants (usually nickel, vanadium, iron, copper, and sodium) with other microspherical particulates high in these adverse elements and very low in activity and which particulates have been in the unit for varying times as long as 60-90 days or even longer. These older catalysts have aged and drastically dropped in performance while simultaneously accumulating these aforementioned deleterious metal contaminants which greatly accelerate catalytically the production of hydrogen and coke as well as dry gas.
As a result, industry has long felt a need to have a means by which the much older catalyst can be selectively removed without inclusion or entrainment of the fresher catalyst in order to reduce catalyst addition rates while at the same time maintaining better activity, selectivity and unit performance. Because of the very small size of these particles, billions of particles are involved, and mechanical separation is impossible even if one could rapidly identify by some means, as for example, color, which particles are old, and which are new.